Propanediol-1,3 is a valuable intermediate product which is used particularly in connection with solvents and plastics.. Industrial scale production is mainly carried out by the catalytic hydration of acrolein with water (see U.S. Pat. No. 2,434,110). An aqueous solution of acrolein is mixed with enough acid to achieve a pH value of from about 0.5 to about 7. In the presence of an anti-oxidant, e.g. hydroquinone, the mixture is heated for 0.5 to 8 hours to temperatures not exceeding 100.degree. C. The reaction mixture is then hydrogenated in the presence of Raney nickel. Variations or further developments of this process relate to the use of ion exchangers as catalysts for the addition of water (see U.S. Pat. No. 3,536,763) and the hydrogenation of the 3-hydroxypropanal in an organic solvent, e.g. isobutanol, in the presence of nickel carrier catalysts (see DE-PS 20 54 601). Through repeated distillation, propanediol-1,3 with a degree of purity of more than 99% is obtained from the hydrogenation product.
However, even after careful distillation turbidity may occur in the pure propanediol-1,3. It is believed to result from the quality, of the acrolein used. By diene synthesis, 2-formyl-2,3-dihydro-gamma-pyrane (dimeric acrolein) can form from 2 molecules of acrolein, the former reacting with propanediol-1,3 forming the hemiacetal and acetal. These aldehyde derivatives are the cause of the turbidity. In some cases, they even impair the further processing of the diol.